1. Field of the Invention
The present invention relates to a superconducting field effect device and method for manufacturing it. More specifically the present invention relates to a field effect transistor type superconducting device comprising a superconducting channel of oxide superconductor which has a larger thickness than the conventional one.
2. Description of Related Art
Devices which utilize superconducting phenomena operate rapidly with low power consumption so that they have higher performance than conventional semiconductor devices. Particularly, by using an oxide superconductor material which has been recently advanced in study, it is possible to produce a superconducting device which operates at relatively high temperature.
One of the most important three-terminal superconducting devices is a field effect transistor type superconducting device (abbreviated as super-FET hereinafter) having a channel of a superconductor formed between a source and a drain. In this superconducting device, a current flowing through the superconducting channel is controlled by a signal voltage applied to a gate formed above the superconducting channel.
The super-FET mentioned above is a voltage controlled device which is capable of isolating output signals from input ones and of having a well defined gain. In addition, it has a large current capability.
Referring to FIG. 1, a typical super-FET utilizing an oxide superconductor will be explained. FIG. 1 shows a sectional view of a super-FET which is similar to the one described in the published European Patent Application No. EP-A-0 533 519.
The super-FET shown in FIG. 1 comprises a substrate 5 of, for example, SrTiO.sub.3, a buffer layer 20 of an oxide, for example, Pr.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-y disposed on the substrate 5, a superconducting channel 10 of an oxide superconductor, for example, Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x disposed on the buffer layer 20, a superconducting source region 2 and a superconducting drain region 3 of Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x disposed at each end of the superconducting channel 10. The super-FET further comprises a gate electrode 4 on a gate insulator 7 disposed on the superconducting channel 10, a source electrode 12 and a drain electrode 13 disposed each on the superconducting source region 2 and superconducting drain region 3.
The superconducting source region 2 and superconducting drain region 3 can be formed of an oxide superconductor different from that of the superconducting channel 10. The gate electrode 4, source electrode 12 and drain electrode 13 can be formed of a noble metal such as Au.
In the super-FET, superconducting current flowing through the superconducting channel 10 is controlled by a signal voltage applied to the gate electrode 4. The Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x oxide superconductor has a carrier density of 10.sup.20 -10.sup.21 /cm.sup.3, so that both of the superconducting channel and the gate insulator should have an extremely thin thickness to obtain a complete ON/OFF operation by a signal voltage of a few volts, which is usually used for CMOS semiconductor devices, applied to the gate electrode 4. For example, the superconducting channel should have a thickness of five nanometers or less and the gate insulating layer should have a thickness of ten to fifteen nanometers or more to prevent a tunnel current, but it should be as thin as possible.
For excellent properties of the super-FET, the thin superconducting channel should be formed of an oxide superconductor Slim having good characteristics and high crystallinity without any grain boundary which may form an unnecessary Josephson junction. In order to) realize this thin superconducting channel, the above super-FET comprises a buffer layer 20 of Pr.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-y oxide having a crystal structure equal to that of Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x oxide superconductor and lattice parameters similar to those of Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x oxide superconductor is formed on the substrate 5 and the oxide superconductor film of the superconducting channel 10 is deposited on the buffer layer 20.
The buffer layer prevents interdiffusion between the substrate and the superconducting channel and cancels inconsistencies of lattice parameters. Therefore, an oxide superconductor film well grows second dimensionally on the buffer layer.
However, the super-FET has such a fine gate structure, as mentioned above, which requires detailed processing steps that are difficult to conduct. Due to the difficult processing steps, the steps for manufacturing the super-FET increases and fabrication yield decreases.